Federal authorities have filed the first criminal charges in the investigation of the deadly terror attack on the U.S. diplomatic compound in Benghazi , Libya , people briefed on the matter said .

Several suspects charged in the armed assault last September 11 include prominent Libyan militia figure Ahmed Abu Khattala , the sources said . Other identities were not disclosed .

The counts initially sought months ago in New York are still under seal , according to the sources .

The investigation of the attack that killed Ambassador Christopher Stevens and three other Americans continues , these people said , as investigators try to build their case against Khattala and others who authorities believe were involved .

If CNN can interview Benghazi suspect , why ca n't FBI ?

Justice Department spokesman Brian Fallon declined to comment as did FBI spokesman J. Peter Donald in New York .

But word of the development comes as the Justice Department has come in for sharp criticism from congressional lawmakers about the pace and progress of the investigation .

Attorney General Eric Holder had earlier this year promised them that the Justice Department would soon make public what actions it would take .

Republicans have demanded more aggressive steps from the incoming FBI Director James Comey .

But people close to the case said the investigation has moved significantly in recent months .

In that time , the FBI has released photos of what it called persons of interest who were present during the attacks , seeking tips from the Libyan public .

It could n't be learned when the United States plans to announce the charges or whether there is any intention to try to detain Khattala or the others .

None of those briefed on the matter would discuss the precise counts .

Benghazi attack timeline

In a recent interview with CNN 's Arwa Damon , Khattala acknowledged being at the Benghazi mission after the attack but denied any involvement .

Benghazi has become a political flashpoint in a long-running battle between the Obama administration and Republicans , who accuse it of not bolstering security before the attack , of botching the response to it and of misleading the public for political gain less than two months before the November election .

The investigation is led by agents from the FBI in New York and has included some members of the Washington field office , who have traveled to Libya to interview hundreds of witnesses , those briefed on the investigation said .

Khattala told CNN that he had not been questioned by either Libyan authorities or the FBI .

U.S. law enforcement officials said it 's not unusual for the FBI not to seek to interview defendants in a case while they collect other evidence .

The Benghazi attack involved scores of militants using rocket-propelled grenades and other weapons .

Stevens took refuge behind a fortified door with heavy metal bars to keep the attackers from breaking in . But they set fire to the mission -- a villa -- with diesel fuel . Stevens was overcome by smoke .

U.S. Rep. Adam Schiff , a Democratic member of the House Intelligence Committee , said in an interview on `` The Lead with Jake Tapper '' that he would not describe the investigation as proceeding rapidly .

`` It 's been frustrating for many of us that it has n't moved faster , but it is a very difficult working environment for our agents , '' said Schiff . `` Gathering evidence in Libya , you might imagine , is very difficult , finding , interviewing witnesses , extremely difficult . ''

But Schiff said there has been progress .

`` We have identified many of the parties involved . We 're still trying to identify what the command-and-control structure would be . A lot of missing pieces still , but we are finally making progress , '' said Schiff .

King : Why Benghazi matters

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FIRST ON CNN : Charges filed against Libyan militia leader Ahmed Abu Khattala , others

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Attack last September killed Ambassador Christopher Stevens and three other Americans

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Authorities would not detail counts as charges remain under seal

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The Benghazi attack has become a political flashpoint in Washington